Because of their various advantages such as small size, high torque, and long life, stepping motors can easily perform digital positioning operations with open-loop control. This fact has led to a wide range of applications to, for example, information devices such as cameras and optical disk devices, and office automation equipments such as printers and projectors.
However, stepping motors have distinct disadvantages such as loss of synchronization during high speed rotation or under heavy load, and lower efficiency than that of brushless motors or DC motors.
One known technology to solve these disadvantages is to prevent loss of synchronization by causing an encoder equipped with a stepping motor to perform operations of a so-called brushless motor in which an energization is switched in response to the position of a rotor (See Japanese Patent No. 06-067259 and Japanese Patent Laid-Open No. 2002-359997). This technology allows high speed operation by switching a current passed through each coil by a signal which is generated by a noncontact sensor built in the motor and is phase-advanced in response to the speed of the motor to make up for a delay in current rise time.
FIG. 8 illustrates torques acting on the motor disclosed in Japanese Patent No. 06-067259 and Japanese Patent Laid-Open No. 2002-359997 when a constant current is passed through each coil of the motor. A current in the positive direction and a current in the reverse direction can be passed through each of two coils, resulting in four different torque curves. Each such torque has an almost sinusoidal waveform and a phase difference expressed as an electrical angle of 90 degree. Electrical angle as used herein means an angle expressed by using one period of the sinusoidal waveform which is 360 degree. When the pole number of a rotor is n, the mechanical angle is expressed as electrical angle×2/n.
An ideal sequential switching of the energization of a motor during its rotation can, on all occasions, achieve a high torque such as T1 indicated with a bold line of FIG. 8A. In such switching, a switching timing of the energization of the coil is determined by a signal generated from each magnetic sensor. The arrangement of two magnetic sensors at an interval of electrical angle of 90 degree permits the switching of energization at the best efficiency.